Stable Time Loop

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"One cannot damage history, because history cannot be changed. [Holds up Clock of Eternity] I went back in time to steal this because history said it disappeared. And history said it disappeared because I went back to steal it. Past, present, future. It's all written in stone, my dear."

Through Applied Phlebotinum, Functional Magic, or some other means, our heroes travel back to the past. In the past, they wind up being responsible for the very events that underpin their own "present." This creates a chicken-and-egg scenario, in which the looping sequence of events has no clear beginning. The result of breaking the zeroth law of Time Travel: do not cause the event you went back to prevent.

This is also the basic premise of how Time Travel would work, according to Albert Einstein. Simply put, even if it were possible to travel back in time, you would not be able to change any events in the past, because they've already happened. No matter what your intentions, everything that you did would only fulfill the past. The only thing that would change is your perception of the events. (Thus explaining Hitler's Time-Travel Exemption Act.)

This trope is actually Older Than Feudalism, since, while time travel is a relatively new concept, prophecy (which is basically information time travel) is not, and Self-Fulfilling Prophecy is the earliest form of stable time loop.

There are two basic types of time loop: a predestination paradox, in which events in the future ultimately trigger events in the past (Sorry Oedipus. And John Connor.) and the even more mind-squirming "bootstrap paradox"note from the classic Robert A. Heinlein short story, "By His Bootstraps" (also called the ontological paradox), in which the time loop allows for the existence of information or objects that have no origin. The classic hypothetical bootstrap paradox is to jump into the future, steal some wondrous gadget, come back to the original time, grab the patent on that gadget and start mass-producing them immediately. Eventually, they become so ubiquitous or so common that you, ten, twenty years younger, show up and steal one. The simplest version is the one where the time machine itself is the product of the stable time loop — the character sees a version of himself pop into existence with a time machine, hand it to him, and press the button, only to be whisked into the past where he hands it to his past self and presses the buttonnote This exact example is also an Object Paradox, wherein the time machine has no past and no future outside the loop, no origin, and somehow never decays or suffers wear and tear. The paradox can be resolved as regards physical objects by noting that such a process would imply the spontaneous creation and subsequent elimination of mass-energy, which violates the 1st Law of Thermodynamics (a principle established 150 years ago to which no exceptions have ever been found).

Examples:

In the Doctor Who fanfic Gemini, this trope is defied by invokingTimey-Wimey Ball: It’s easier for the Daleks to change the past than it is for anybody else, so the military is trying to create super-soldiers that can change the timeline as easily as the Daleks can.

My Little Pony: Friendship Is Magic has ''Que Sera, Sera''. Celestia's ability to know the future is finally explained after she summons Twilight Sparkle and her friends to defeat a great foe — an ancient dragon who has awoken and gone on a rampage, but Celestia knows that it is all according to plan — and indeed, the rampage was a collaboration between the dragon and Princess Celestia, without even Princess Luna's knowledge. The reason is that the dragon is Spike, several thousand years older, and he uses his firey breath to send himself and the Mane Six back over two thousand years into the past. Twilight at first worries about changing the future, but eventually realizes that they are permanently stuck in the past and relaxes. Eventually she has two children, naming one of them after Celestia, and the other after Trixie, though her sister mispronounces Lulumoon as "Luna-moon". When Discord shows up, she and the rest of the mane six expect the princesses to show up... but they don't, because they're still foals. Namely, Twilight's. And the Elements of Harmony don't even exist yet, the only weapon that can beat Discord. Or at least, they don't exist in the form of the elements... In the end, Twilight and her friends have to sacrifice themselves and BECOME the Elements of Harmony, entrusting Spike to help Luna and Celestia, and then eventually become the monster that sends them into the past in the first place, several thousand years down the line. The story ends with Spike having reverted to his child-like form and finally reuniting with Celestia, who has just had to send her own mother to her death.

Harry Potter and the Methods of Rationality has four of these to date, the most notable one being when Harry pulled a prank on himself using a Time Turner, an Invisibility Cloak, two pies, and several sheets of parchment.

His attempt to use Time Loop Logic (see Real Life below) as a manually-performed perfect algorithm was... less than successful. The output: DO NOT MESS WITH TIME

Finally, there's the prophecies. Dumbledore listened to all of them, and then set the events in motion that would lead to Harry's birth and how his personality developed.

Kyon: Big Damn Hero has these every few chapters, and so many that any unresolved ones are offering Kyon protection from the IDSE. As soon as he resolves the last one...

Yabba Dabba Joes — Destro went through almost three dozen agents trying to kill members of the Joe team in their cribs before finally accepting that time travel in the Joe-verse doesn't allow changing the past.

Paradox has a stable time loop despite the name. Shampoo strands Ukyou and Ryouga in the past, where they become the real parents of Ranma who is stolen at birth by Genma.

One Harry Potter fanfic had a four year old Harry being sent back in time to when his parents were newly married. In the end Harry gets sent back to the present, completely forgetting everything that happened. Meanwhile in the past, Sirius convinces James to make Peter his and Lily's secret keeper so they won't be killed and the future Harry came from will never be. ...yeah.

In another Harry Potter comedy (Harry Potter and the Sword of Gryffindor by cloneserpents), Hermione steals a time turner for the purposes of "kinky sex" that will also hurt Death Eaters. This is explained by Hermione at the time saying that she sort-of got it through a time paradox, but not to worry about it. Later, Harry is sent to put it back in the Department of Mysteries at the same time as stealing it in the first place. On the way, he runs into Mad-Eye Moody, who says that the DoM is being guarded after the events of Harry Potter and the Order of the Phoenix and that he should probably not venture in there. Upon exiting in failure, Hermione suggests that he just give her the one that he had to put back. This leads to Harry having a Logic Bomb moment along the lines of "But you gave this to me after traveling through time... and I just gave it to you... where did it come from?!"

The Marvel-verse fanfic Dreams of the Waking Manis all about one giant stable time-loop. In the far future, Deadpool helps Cable and Hope to return to the present, which causes a chain of events that influences the entire Marvel universe and ensures Deadpool will always be in the Future to help Cable and Hope to get back to the present.

The Buzz Lightyear of Star Command fic One Hundred Dayshappens because one long time-loop. Warp's canonical death-stunt sets up Buzz and Mira to work together as teammates, thus giving them the opportunity to know each other very well. Then, Warp's revelation of his parenthood drives the pair to marry and avoid a scandal, thus allowing him to be born in the first place. At some undefined point in the future, Buzz and Mira send a baby Warp back through time (the how remains unexplained) and close the loop, as remembered by Warp in the climax.

The entire premise of the Facing the Future Series is dependent on one of these. When Future Danny and Sam travel back in time to fight Dark Danny, present day Sam is driven by the revelation that her future self is half ghost to gain ghost powers of her own, thus becoming Danny's new partner.

The Pony POV Series has a rather complex one show up in the sub-arc following the conclusion of Dark World: When Twilight/Amicitia ascends, Cadence warns her that her greatest enemy awaits at the dawn of time. So she goes back then, where Mortis warns her that they're destined to one day fight each other as well, before helping her evade a sneak attack from a younger Cadence, who's convinced that magic is inherently evil and wants to kill Amicitia to prevent its existence. After Amicitia barely defeats her, she then hops back in time a bit and tells Mortis to warn her, before going on to traverse her own timeline to be the Benevolent Interloper and shield her mortal self from Nightmare Eclipse/Paradox's influence, thus insuring her own existence. And then she makes sure Razzaroo's Apocalyptic Log survives G3's Cosmic Retcon and sends it to the spirit world, where Razzaroo would then use it to nearly become The Magician Alicorn and challenge Dark World Twilight for the spot in the first place. Notably, Mortis at one point warns her not to do this too much, as it becomes complicated.

Also, there's implication that Amicitia had to complete this because she and Paradox were two alternate futures Twilight could have become, and by closing the loop, she locked Paradox's defeat in stone.

In Suzie's Adventure World, this is said to have happened with a card exchanged between Suzie and Ryo where the one Ryo gave Suzie is a double of the one she gave him years ago, and she gives it to him in Adventure...

Suggested in the Empath: The Luckiest Smurf story "Days Of Future Smurfed" that Empath is supposed to create the memory artifact that will fall into the hands of Peyo, leading to his creation ofThe Smurfs, while Empath marries Smurfette, has a child through her as well as a grandchild and great-grandchild, the last of whom will become Traveler Smurf, who will travel back into the past to give Empath visions of the future that will lead to the creation of the memory artifact.

Deconstructed in the Ranma/Sailor Moon crossover No Chance For Fate in the first chapter until a random factor breaks the loop, setting up the rest of the story with Sailor Pluto hell bent on preventing the situation that caused it in the first place.

Jewel of Darkness has a small one in the Rivalry Arc. Midnight goes to steal the Clock of Eternity, only to find it already gone. After the ensuing confrontation with Warp and trip to the canon universe, she ends up back in the museum five minutes before her first arrival, allowing her to steal the Clock before she even shows up.

A minor example from Rose Of Pollux's fanfics concerning Jamie's nickname. As it turns out, the Doctor gave Jamie his nickname at their first meeting (from Jamie's perspective, which occurred when he was about two months old). Which he only knew because that was how Jamie introduced himself at their first meeting from his perspective.

A reverse form occurs in Paradox or Pair of Twins? when Xander is sent to the future and has sex with the DeeDee twins. Xander later mentions they did things he had never even heard of before, while in the future the twins realize Xander is the guy who literally wrote the book on sex. In other words, they did things they learned from his book which he wrote after they did said things to him.

Child of the Storm uses one of these to explain why Professor Xavier couldn't tell Harry about his relationship to Jean — fifty years before the start of the story, he met a time-traveling version of Harry from two years into the story's future, who told him the things he had to do to ensure the closing of the loop (and thus avoiding a dangerous paradox), including swearing everyone present to secrecy until such time as Xavier could give present-day Harry a letter from his future self, that he remembered receiving from Xavier. Discussing this sequence of events gives everyone a headache.

Yin And Yang Series: A few. It's only natural with a time-controlling alien as a major character.

The Monkey D. Haru Series: In Prisoner of Azkaban, it's revealed that the reason why the Wizarding World knew Harry Potter was alive during his ten-year long disappearance is because Haru used his Portal-Portal powers to send a picture of himself back in time to Dumbledore.

Music

In live performances, the Flight of the Conchords song 'Bowie' is usually preceded by a description of Bret and Jermaine travelling back in time and meeting David Bowie, to whom Bret plays his Bowie's own songs, and even leaves an "easy to play Bowie song book".

The Black Sabbath song "Iron Man" is about a person who travels through time "for the future of mankind" only to find that the world is destroyed in an apocalyptic event. Deciding to return to his present to warn the people of the coming disaster, he gets "trapped in a magnetic field" which turns his skin into metal. Thus, when he warns the people of the present, they are frightened by his appearance and too afraid to listen to him. Then, out of frustration that no one heeds his warnings about the forthcoming apocalypse, he causes the apocalypse.

"One For the Vine", on the Genesis album Wind and Wuthering, tells the story of a soldier deserting from an army led by a messianic leader. The deserter finds himself on an icy waste populated by primitive people, who see him as a messenger of God. He reluctantly takes the role simply in order to help himself get home, but ends up becoming the very messiah from whom he fled. As he leads his army into battle, he sees one soldier run away from the host, and vanish...

Show Of Hands song The Bet is about a man who finds ten grand next to a car crash, takes it, bets on horses with it, wins ten grand and... yeah, you see where I'm going with this.

Possible example in the Cygnus X-1 duology by Rush: the narrator is pulled into the black hole known as Cygnus X-1, and arrives in a world ruled by the Olympian gods which may or may not be our own world in the distant past. He ends the war between Apollo and Dionysus by telling them his story. They dub him Cygnus, after the black hole through which he entered their word. He becomes a god, presumably the mythological figure the constellation Cygnus, and thus the black hole Cygnus X-1, is named after.

Pinball

Loony Labyrinth and its sequel, Mad Daedalus, forms a Stable Time Loop with regards to the invention of the Loony Machine: the player travels to 2,000 B.C. with the Loony Machine, which Daedalus examines. After the player returns to the present day, Daedalus eventually (re-)invents it. However, Daedalus cannot power it because his assistant lost the Loony Stones that power it, so Daedalus places the machine in the Labyrinth. it is excavated several thousand years later, and the player finds the Loony Stones, then uses the Machine to travel back to 2,000 B.C....

The audio drama "Flip-Flop" takes this to a rather confusing extreme: Two time loops that feed each other. It's presented on two discs, a "White disc" and a "Black disc", and they can be listened to in either order (or indeed in a continuous loop), as each one follows a different timeline. To summarize: On both discs the Doctor and Mel arrive to find the planet Puxatornee on Christmas Eve just before midnight in a terrible way: On one disc, a radioactive wasteland, on the other controlled by a hostile alien species. They are forced to go back in time to prevent it, and go back to Christmas Day to find the planet worse: On one disc, controlled by an alien species, and on the other a radioactive wasteland. They are then forced to go back to Christmas Eve before they arrived, and leave just before their other selves arrive on the planet, beginning the adventure on the other disc. In essence it's two unstable time loops, each leading to the other one.

In the Companion Chronicles audio drama "Tales from the Vault", a time-release capsule in UNIT's Museum of the Strange and Unusual reveals a message from the Doctor's companion Steven Taylor warning of an alien threat. When it gets Lost in Transmission WO Charlie Sato, against the orders of Captain Ruth Matheson, tries using a related item in the Vault to get more information in the process releasing the alien threat. After it's been defeated, he asks Matheson why the time-release capsule had opened today in the first place. She replies that the Doctor presumably read the report she's about to write.

The Companion Chronicles drama "Return of the Rocket Men" begins with Steven on his twenty-first birthday, being tortured to death for fun by a Space Pirate before being unexpectedly saved by another pirate named Ramirez, who he then sees get shot by his Bad Boss. Older, experienced and now travelling with the Doctor, he ends up unexpectedly on the same planet on the same day, ends up in the suit belonging to Ramirez, and realises he now has to save his own younger self, even if it means him getting killed.

The opening story of The New Adventures of Bernice Summerfield: Volume 2 The Triumph of Sutekh has Benny discover the Doctor in a Martian pyramid fighting Sutekh for possession of his mind. Then he dies. The final story has the Doctor, battling Sutekh in the far future, disappear as Sutekh attempts to take over his mind. He then gets pulled out of the loop by Isis, but Sutekh is left to repeat it forever.

Continuum is an RPG where the characters' entire goal is to make sure stable time loops work out.

Planescape's Faction War features a double time loop. Considering that the person stuck in it tried to overthrow the Lady of Pain, he had it easy.

Rulified by the German RPG The Dark Eye in which time travel follows a simple law: you cannot change the past, as it had already happened and you'll just end up doing what you did to create the present you're currently living in. If by some chance the hero does discover some hopelessly contradicting action, be prepared for time to heal itself. Oh, and the universe has wardens against such misuse, too.

An Imperial warship picks up a distress call from an Imperial vessel under heavy attack, and goes to respond. When it arrives, it finds no Imperial ship, but the warship itself comes under heavy attack... and sends out a distress call. Thanks to the ability of the Warp to mess with time, the ship went to its destruction answering its own distress call.

The ork warlord Grizgutz and his army, after setting off into the warp, arrives shortly before they left off and decides to hunt down and kill his previous self so he can own a spare of his favourite gun. The confusion results in the war-band being stopped in it's tracks.

Magic: The Gathering: The Tarkir block sees Sarkhan Vol go back in time to save the spirit dragon Ugin from being killed by Nicol Bolas. The result turns his world from ones where dragons don't exist to one where dragons rule the place...and he was never born.

Visual Novels

In Ever17, the main character (revealed to actually be a 4th-dimensional being known as "Blick Winkel") travels back in time from 2034 to 2017 to save two other characters from certain death, only to find that if he immediately reveals their survival to the others, that will create a Temporal Paradox preventing him from coming back in time in the first place—so instead he is forced to hide their existence and manipulate the others into setting up the event in 2034 that results in him being "summoned" in the first place.

Fate/stay night. The swords that Archer carries. Shirou can only create a weapon he's seen (most of the swords created in Unlimited Blade Works come from weapons he saw in Gilgamesh's Noble Phantasm, Gates of Babylon). However, in the case of his two main swords, Shirou learned to create them from seeing his future self wielding them. Shirou doesn't have to become Archer in the future for this to be valid, though; he just has to see the swords. In other words, whether or not it is a stable time loop is less important than the fact that it could be.

In Nine Hours, Nine Persons, Nine Doors, this is one possible interpretation of the ending: Akane worked her way through the Nonary Game 9 years in the past, transmitting the answers into Junpei in a possible future. When she reaches a puzzle she can't solve, she explores through possible futures until she figures out how to lead Junpei into one where he faces the same puzzle. He's able to solve it, and transmit the answer back to her, allowing her to avert her own death. But after this incident, she has to set up the second Nonary Game that Junpei finds himself in 9 years later, completing the loop. note The alternative theory is that Santa is trying to Set Right What Once Went Wrong, while Akane is projecting an image of herself into the future throughout the first game.

One occurs in the path where the old woman does not die — upon nearing the end of the path to Phi's ending, Sigma and Phi go back in time and save her, thereby setting up the timeline in which she lives.

One depends on the player's actions, but Phi will betray you at a critical juncture, in revenge for you betraying her at that same juncture in an alternate branch. It is extremely likely that you will play this branch before the branch where you actually commit this betrayal, and only do the latter at all because the first spoiler is blatantly railroading you into it, because without that as motivation, it would be completely ridiculous, cruel, and out-of-character. That said, if you did decide to be ridiculous and betray Phi first, then no time-loop is involved.

In Zero Time Dilemma, this is half of Zero's goal. He needs to create one so that he and his twin sister are born under specific circumstances to make them both ESPers.

Time Hollow is one BIG Time Loop which is both stable and constantly shifting. The overarching plot is one huge example due to the protagonist sending himself hints and clues at the end of the game to his startgame self, but the events of both the past and present during certain periods is in constant flux, even though due to the looping nature, that flux is always in its own stable loop.

Demonbane has a time loop as its central plot point: the entire universe has been time-looped an unknown (but very large... when an Outer God loses count, you know it's a big number) number of times, and the antagonists often refer to the looping as "the infinite spiral" and the "Wheel of Fate". The game's good endings involve the heroes breaking the loop. The ending and sequel imply that Nyarlathotep, the supposed architect of this time loop, was itself a pawn in an even bigger, trans-universal time loop, perhaps one orchestrated by Elder God Demonbane itself.

The True Ending of Steins;Gate reveals that one of these has been ongoing the entire game. The whole plot starts with Okabe seeing Kurisu dead in a pool of blood, causing him to accidentally send the first D-Mail and reach the Alpha world line, where most of the plot takes place and Okabe gets to fall in love with the now living Kurisu. However, in the end, Okabe returns to the Beta world line, where the story started, but has to save Kurisu from her death at the beginning and stop World War III from breaking out. To do this, he lets himself get stabbed in Kurisu's place and then knocks out Kurisu and uses his blood to to recreate the scene his past self originally saw. Thus, Kurisu lives, and Okabe still goes through the events of the visual novel which ultimately lead to him working to save Kurisu.

Web Original

This is essentially one of Dr. Insano's backstories as part of The Spoony Experiment: Insano is an alternate-universe version of Spoony, who has grown so angry with the Final Fantasy franchise that he wants to go back in time to erase it from existence. Since being able to travel through time would require him to study science for decades, he decides to create a time loop just like that of the originalFinal Fantasy by studying science, travelling back in time and then obtaining all the knowledge he needs from his future self.

In Red vs. Blue, Church creates an uncountable number of these as he fails his objective each time and keeps trying. Local Cloud Cuckoolander Caboose makes the following unintentionally profound statement when Church talks to him about his experiences with the timeline: "Time LINE...? Ehh, time isn't made out of LINES. It is made out of circles. That is why clocks are round!"

In Illo Tempore contains at least two stable time loops across four millennia.

The Flash game No Time To Explain. Your character is chilling at home when a future version of himself appears out of thin air, warning you of imminent danger. Seconds later, a giant crab grabs him and carries him away, leaving you to use his weapon to save your future self. After defeating the crab, your future self opens a time portal back to the beginning of the game before dying. You travel back to your past self's living room, and try to warn him of the danger your future self warned you about. Seconds later...

Robutt a robot is trapped in a time loop wherein it constructs itself out of junk, sacrifices its battery to power the new version, which gets in a time machine and goes back to do it again.

In We Are Our Avatars, Komatsu and the others teach a few of their moves to a younger Toriko, setting Toriko's path in becoming a great Gourmet Hunter. In fact, Toriko learned his Knife and Fork techniques from Grey Fullblaster.

In Chronicle of the Annoying Quest, when the party reaches the Caverns of Time, Guy accidentally activates a rune that displaces him in time from previous points in the story, including his original warlock apprentice outfit, his tier 1 set ghost form, and his tier 2 set undead form. After a few confused remarks by the past versions, Guy casts a spell to send them all back. What makes it an example is present!Guy's reaction to the situation:

"Oh, so this is when all that happened! I thought it was just a recurring dream."

Starwalker: Starwalker's drive allows her to travel through time. This gets the attention of some Space Pirates who take over the ship. They force her to use her drive to go to the Solar System. Using the ability to travel in time, Starwalker gets them to the appointment 40 years early. They return to the present. However, the ripples of their passage causes strange measurements to be made of our sun. This gives Dr Cirilli the idea that a star step drive could work. In the "present" they meet the person who hired the pirates. She turns out to be the avatar for our sun. She hired the pirates to end the Starwalker experiment because the star step drive has the side effect of Star Killing.

random_nerd: Completely ridiculous, I say! Everyone knows the Titanic sank from the added weight of all the time travelers trying to stop it from sinking.

Happens in Tribe Twelve during the November 11th Livestream: on the night Noah is supposed to be taken and forced into the Collective, Firebrand appears in the corner of his bedroom and transports him to his own closet on August 1st of the previous year. Noah begins pounding on the door and shouting, dropping the rubber ball with the Observer symbol (which he found in his closet) as he teleports away — leaving the ball to be discovered by his past self, who was alerted by the noise from the closet. Present!Noah then finds himself in the hallway outside the hotel room where he stayed on his birthday the previous year; he opens the door, frightening his past self (and giving him his first sighting of Firebrand) as he's abducted by the Slender Man. Present!Noah is then returned to his bedroom — in time for his past self to get his second sighting of "Firebrand" in the corner of his room as he's teleported away.

Troll Science teaches us how to make use of a stable time loop to acquire infinite money. Just spend your entire life saving every cent you can, hop in your time machine, and travel back to give your younger self all your savings and the time machine. Repeat as necessary. Since you've already been doing in indefinitely, all you have to do is wait for your future self to bring you that time machine and a fantastic pile of money.

A web roleplaying forum had a story of myths of a terrible army of monsters known as the Zanik, who were supposedly born from a devil equivalent and forced the people of the world to climb on the back of a god equivalent and fly off into the sky to escape them; the myths, however, give no answer for what happened to the Zanik after that: they essentially vanish. An evil witch, discovering that the flying island exists (the myths being the supposed reason why there's a flying island in the shape of a dragon in the world), ultimately provides the reason that the Zanik vanished from the myths; she travels back in time and brings them to the present to wreck havoc, ultimately causing their complete vanishing from the myths that she studied that inspired her to go back and do that in the first place...note Unfortunately for the Zanik, they very swiftly discovered that the forces that could oppose them in the ancient past and the forces that could oppose them in the 'present' day were light years apart, and were wiped out to a man.

Protectors of the Plot Continuum agents Rina Dives and the Reader experience this: the Reader discovers the human-turned Time Lord Rina is actually her old nanny from Gallifrey who helped her escape the Time War, which led to her finding the PPC and subsequently meeting Rina, who was still basically a child and had yet to run away and end up on the Reader's home planet. The Reader ends up desperately trying to not let anyone know of what will have to happen in order to make sure time doesn't collapse. It leads to a lot of stress for both parties when they discover the other's deception.

In the Mega Man games episode of The Angry Video Game Nerd the Nerd decides to go back in time to see past versions of himself. He goes back to 2007 and sees himself just having reviewed Independence Day on PlayStation, and tells him to review some crappy games based on The Simpsons. He goes back to 2006 and sees himself in his review of A Nightmare on Elm Street as "The Angry Nintendo Nerd" and convinces him to change his name to "The Angry Video Game Nerd". Finally he goes to 2004 and tells the version of himself that just finished ranting on Dr. Jekyll & Mr. Hyde to keep working on reviewing games he thinks are of poor quality.

Parodied in one video by Chris Ray Gun. It begins with Chris rolling on-screen on a fast-moving swivel-chair, which causes him to immediately crash into a wall, and receive amnesia. He then tries to figure out what happened before he got amnesia, and immediately finds a YouTube video by Zarna Joshi, which becomes the topic of that Episode as Chris gives every criticism he has of that video. When the video ends, Chris regrets wasting his time watching it, and promptly builds a time machine out of his swivel chair, intending to use it to go back in time and prevent himself from ever watching it - and it is revealed that his crash at the start of the video was the result of him time travelling back to the past at the speed of light.

Real Life

You never thought you'd see it, but some non-trivial physicists are wonderingaloud whether the future is actively trying to scuttle the LHC research project. The string of mechanical difficulties, they say, may be more than mere chance: Whether through [future] human intervention or the universe itself exercising some form of upstream-acting Ontological Inertia, the future is trying to make sure we don't mess with Higgs Bosons. Loopy? Literally. Crazy? Probably. Impossible? Well...

In terms of the theory of relativity, time travel would take the appearance of a closed timelike curve, which is a series of events returning to its starting point — such as time travel returns to the past. That leads to the inevitable paradoxes, but some solutions claim that if time travel is possible at all, it's only possible in a stable form: the only form of time travel possible generates one.

While that may seem supremely useless — what good is a time machine if it can't change anything? — that's not really true. A computer that could only build Stable Time Loops could run algorithms like "open time communication channel to 5 minutes into the future, receive answer from future, close channel, check answer (which takes 5 minutes in this example), if correct, send answer over the communication channel that was now just opened, if incorrect send different answer to past over the same communication channel", and then consistency would force the algorithm to return the output to some given puzzle. This is known as Time Loop Logic. Receiving an incorrect answer from the future would cause a paradox, and thus is impossible. Therefore either the answer will never arrive, or you will have created an ontological paradox, fabricating the answer from nowhere. The step of checking the answer is required for the algorithm to work.

Less esoterically/more science-fictionally, a time machine that can't change anything would also be just dandy for observation, thanks. (Better, really, since you don't risk stepping on the Butterfly of Doom.) Video from 20,000BC anyone?

Some theories of particle physics hold that antiparticles (particles with the opposite charge and parity of a "standard" particle) travel backward through time. It's also widely believed that particle-antiparticle pairs randomly pop into existence from the Quantum Foam, and then feel their mutual attraction and annihilate each other moments later. It's also possible to see this as a single particle traveling in a loop through time: Forward as a regular particle to annihilate with its antiparticle, then backward as its antiparticle to annihilate with its standard particle, and repeat.

A few theories state that, assuming time travel is possible, the big bang could have occurred by sending a time machine back to the starting point of the big bang, which would be a void before 'existence'. The sudden appearance of matter would then trigger the big bang, causing us to exist and said time machine to be constructed.

There is another theory that says that if time travel is possible, it is only forward time travel — theoretically caused by time dilation when approaching the speed of light. The other way says that time machines are possible but that you can only travel to when the time machine was first activated and any time after that, but not before. That would explain why they aren't hundreds of time travelers at every major event in history... yet.

The first isn't a theory as per the use of the term in the rest of this page, though some of the underlying physics has yet to be proved. It's how we are currently time travelling, and the change in rate can be demonstrated empirically by looking at the clocks on GPS Satellites and seeing that time passes differently on them and the surface.

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